Apparatus and process for the making and coating of hollow bodies

ABSTRACT

Metallic hollow bodies such as cans or other containers, particularly non-ferrous cans such as aluminum, are electromagnetically suspended and coated on their outside and inside surfaces simultaneously by applying, in an electrostatic field wherein the metallic hollow body is grounded, a resinous powder coating material, and whereafter the hollow body is subjected to a heat treatment, to result in formation of a continuous resin film coating on the surfaces of the hollow body. In the case of non-ferrous (e.g., aluminum) cans, cans susceptible to electromagnetic suspension are manufactured from aluminum (or other non-ferrous) sheet by adhering iron mesh to the cup-bottom during the cup-forming step.

United States Patent 19'] Becker et a1.

' n11 3,815,535 [451 June 11,1974

Kleindrettle, Trossingen, both of Germany W. L. Schwaab Lackfabriken KG,Inhaber: Gebruder Becker G.m.b.l-l., Weingarten u. Karlsruhe, GermanyFiled: Dec. 14, 1972 Appl. .No.: 315,251

[73] Assignee:

[56] References Cited I UNITED STATES PATENTS 1,958,765 5/1934 Perkins..113/120 A IMPACT EXTRUDER 8/1950 Mapes et a1 113/120 R 3,406,55410/1968 Frankenberg 113/120 R Primary ExaminerRichard J. HerbstAttorney, Agent, or FirmBurgess, Dinklage & Sprung [57 ABSTRACT Metallichollow bodies such as cans or other containers, particularly non-ferrouscans such as aluminum, are electromagnetically suspended and coated ontheir I outside and inside surfaces simultaneously by applying, in anelectrostatic field wherein the metallic hollow body is grounded, aresinous powder coating material, and whereafter the hollow bodyissubjected to a heat treatment, to result in formation of a continuousresin film coating on the surfaces of the hollow body. In the case ofnon-ferrous (e.g., aluminum) 'cans, cans susceptible to electromagneticsuspension are manufactured from aluminum (or other nonferrous) sheet byadhering iron mesh to the cupbottom during the cup-forming step.

3 Claims, 7 Drawing Figures VERTICAL ELEVATOR OVEN COOLING VENTING ZONEzone 1 COATlNG WASHING MACHINE APPLICAHON CAN OFFSET OUTER- Y FlNlSl HNGPRINTING COATJNG r INAL SINTERING a 0 OVEN PRE DRYING OVEN I FATENTEDJUH1 1 I974 331 5535 SHEET 3 OF 3 FIG. 38,. FIG. 3B2

IMPACT 39 mgggggs EXTRUDER CAN APPARATUS AND PROCESS FOR THE MAKING ANDCOATING F HOLLOW BODIES This invention relates to a process for themanufacturing and coating of hollow bodies such as cans, containers andlids. More specifically, the process of the instant invention relates tothe manufacture and coating of aluminum cans, e.g., aerosol cans.

It is, of course, conventional to provide hollow metallic bodies, suchas containers in general, cans and lids, with a lacquer coating on boththeir inside and outside surfaces for protection from aggressivelyacting filling materials or for protection from external influences,and/or for optical and decorative reasons. The application of suchlacquer can proceed conventionally through an electrostatic process.This process is based on the fact that electrically charged lacquerdroplets move in a field of high electric strength along electrostaticforce lines toward the grounded hollow body surface andare dischargedthereonQBecauSe the electrostatic potential difference is graduallyreduced with increasing lacquer coat thicknesses, the higher potentialdifference obtaining at unlacquered places of the body automaticallycauses a relatively uniform lacquering over the entire body surface. Toensure that the lacquer coating will meet all requirements, care must betaken that the lacquer adheres firmly to the surfaces to be protected.ln the past, it has been necessary, to serve this purpose, to treat thesurface of the goods to be lacquered, in comprehensive pre-treatmentsteps, to free it of impurities such as oxides, fats and dirt. Dependingon the nature of the metal of which the container is made, it must besubjected to cleansing, defatting, and finally, a phosphating step.After this pretreatment, there is normally effected, in the case ofhollow metal containers, lacquering of the inside surface of thecontainer and, following this, the inside lacquer is annealed or bakedin a kiln or furnace. Thereafter, the hollow bodies, suspended on metalprongs, are lacquered from the outside and pre-dried in a furnace. Afterthe article is imprinted, according to means conventional in the canningarts, there must be applied another, colorless, lacquer coat to protectthe printing. This lattercoat can be applied weton wet anddriedtogetherwith the predried article, or it can be applied to the finallydried, e.g., annealed, article and burnt in separately. The final dryingis also usually effected in a run-through oven.

Apart from the great number of steps required in a prior art processes,such processes are'subject to additional disadvantages. Thus, it canfrequently happen that the lacquer coat on parts of the article whichare difficulty accessible due, e.g., to insufficient cleaning, defattingor phosphating, adheres only partly, or not at all, to the containersurface. ln the subsequent heating step inthe oven, there can thus beformed bubbles, protrusions and pores at such places. Also, theevaporation of the solvent from the lacquer, which takes place upon theheat treatment, can deleteriously affect the pore density of the lacquerfilm. These disadvantages are substantial because it is possible, ifimperfections exist on the inside of the container, that the containedgoods are in direct contact with the metal wall of the container.Corrosive goods can thus result in undesired chemical reactions. Due tothis,'the lacquer film, starting from a very small vulnerable (i.e.,insufficiently lacquered or porous) spot exposed to corrosive containedmaterials, peels gradually from the container wall, and

this can, forinstance in the case of spray cans, e. g., aerosol cans,lead to plugging of the aerosol can spray nozzle in which small lacquerparticles deposit themsleves. Because the spray nozzle cannot be cleanedfrom the outside, the remainder of the contents of the can can no longerbe sprayed out, and is thus rendered worthless. In extreme cases, it canhappen that corrosive contents which are in contact with the containerwall for prolonged periods, can eat through the entire wall (which isusually quite thin) and thus, lead to spillage of often inflammablecontained materials. Apart from damaging the container wall, thereactions between the contents of the can and the container wall can, ofcourse,also deleteriously affect the contained goods. Particularly inthe case of canned foods, the danger of, at best, spoiled taste, or, atworst, complete spoilage, of such food stuffs, is substantial. Forinstance, the oxidation of fats, i.e., rancification, .is accelerated ina high degree by heavy metal ions.

To mitigate the. above-mentioned drawbacks, it is often necessary totailor-make the inner lacquer coating to suit the requirements ofparticular aggressive or corrosive goods which, apart from the extraeffort required, has the disadvantage that the so-treated container canoften be used for only one kind of contained goods. i

Another feature, of prior art processes for making and coating cans,particularly in'the drawn-and-ironed can-making processes currently ofcommercial prominence, which has proven a draw-back, is the inability ofsuch processes to handle ferrous and non-ferrous materials on the sameline without extensive modificationsof the equipment and/or process used(see, e.g., Modern Packaging, June 1972, page 29).

The present invention substantially eliminates the drawbacks of theprior art processes and, specifically, greatly reduces the pretreatmentsteps conventionally required prior to lacquering, avoids the necessityof two heat treatments, and eliminates the need for a protectiveadditional lacquer coat to protect any imprinting on thecan. Inaddition, the process of the instant invention results in substantiallycomplete avoidance of pores or holes in the lacquer film, thus makingimpossible contact between the container wall and'the containedgoods-and rendering cans treated by the instant invention particularly'suitable for use when canning food-stuffs. Still further, the process ofthe instant invention cuts down or eliminates the loss of lacquerexperienced in .conventional processes and apparatus, so

that, apart from providing a simpler process, the instant inventionprovides a more economical process as well.

In specific and preferred aspect of the invention, aluminum cans aremanufactured and coated in a sequence of steps which has proven highlyefficient not only for the large-volume production of finished andimprinted aluminum cans from aluminum sheet, but which are capable ofproducing both steel and aluminum cans on the same line without anyadjustment being necessary; i.e., the same equipment can be used withoutmodification to produce, alternatingly, steel and aluminum cans.

Essentially, the instant invention comprises an apparatus and a processfor manufacturing an electromagnetically suspendable metal cup frommetal sheet, electromagnetically suspending the cup, e.g., aluminum can,to be treated in an electrostatic field, introducing into saidelectrostatic field a spray of finely divided solid resinous powderparticles, grounding said hollow body to cause said particles to'adhereto the surfaces of the hollow body, and then subjecting the hollow bodyto a heat treatment whereby a continuous resin film covering the entirehollow body surfaces is formed. When the metal can is non-ferrous, e.g.,made of aluminum, this invention comprises forming a cup from aluminumsheet while simultaneously adhering a strip of iron webbing to cover atleast part of the bottom portion of the cup, or imbedding such ironwebbing into the bottom portion of the cup, thereby rendering the rawunfinished cup electromagnetically suspendible.

It has been found that, by use of the invention, a homogeneous, uniformand pore-free coating is obtained by virtueof the electrostatic behaviorof the coating particles in'relation to the container to be coated andthat ferrous and non-ferrous cups can be processed alternatingly on thesame treatment line, without modification of the equipment used.

In contrast to electrostatic lacquering, wherein fluid media are sprayedonto the container surfaces, which media adhere to the surfaces becauseof binding agents contained therein, the solid powder form resinparticles brought into contact with the container surfaces in theinstant invention principally adhere to the container surfaces becauseof the potential voltage difference caused by the electrostatic fieldand not because of any chemical binding agents. Only upon the subsequentheat treatment of the instant invention are the resin particles meltedtogether to form a continuous, closely adhering film surrounding allcontainer surfaces. Because no organic solvents are involved, completecontinuity and absence of pores are obtained in the instant invention.It will be understood that the heat treatment step carried out inconventional lacquering operations, is carried out for the entirelydifferent purpose of removing the solvent contained in the lacquerthrough evaporation, to dry the lacquer thereby and to adhere it morefirmly to the container surface, which, however, often results in poreformation, as explained above.

The use of resin powder in place of the liquidlacquer of prior processesmakes. possible a substantial reduction in the pre-treatment required inlacquering processes. Thus, when proceeding in accordance with theinvention, a defatting of the container, e.g., cans, is all that isrequired and the separate cleaning and phosphating steps required byprior art techniques are eliminated. Further, even the defattingpretreatment step required in the instant invention is not as criticalas it is in prior art processes; for instance, streaks of fat that mayremain on the container surface after defatting, which have to beremoved if the container is to be subsequently liquid-lacquered bythorough rinsing or spraying, do not disturb film formation whenproceeding in accordance with the instant invention because the powderyresin particles melt together to form a continuous film thereoverwithout being affected by such fats, streaks or smears.

Another substantial advantage of the process of this invention and useof the inventive apparatus is that it is no longer necessary to storethe freshly lacquered goods under ventilation, which was required in theprior art lacquering processes. It has been found that powder, ascontrasted with'liquid lacquer, is adapted in its freshly applied formto continuous-line processing,

' easily tolerates the intense heat treatment in the oven (thussimplifying the construction of the oven and the length of time thearticle needs to be in the oven), thus offering very substantialadvantages in processing. in addition, the powder-coated articles madein accordance with the invention facilitate the application of graphicarts designs on the coated can: the imprinted matter is melted into theresin film, in the heat treatment step of the instant invention, so thatthe prior art requisite of applying an additional protective lacquercoat and heat treatment of same are no longer necessary. Still further,the process of the invention does not require complicated ventilatingequipment, because the heat treatment of the powder-treated cans cannotlead to the escape of solvents which may be inflammable or hazardous tooperatives.

As set forth supra, there is little loss in resin particle spray in theinstant invention, contrasted with the substantial loss of lacquer sprayin liquid-lacquer prior art processes by way of spray which is drawn offthrough the air purification equipment requisite in prior art processes.In the instant process, excess resin powder, i.e., particles which donot reach the can to be coated, are easily collected below the coatingapparatus by way of, e.g., a trough disposed within the electrostaticfield, and can be easily recycled to the spray nozzles. Other advantagesare inherent in the specific properties of the resin material applied,viz., its great resistance to-many chemicals (thus leading to universalapplicability of the finished goods to hold all types of containedmaterial), its low density, its high elasticity and hardness.Particularly, it is possible, because of the high elasticity of theresin film, to deform bodies so treated after they have been coated,without leading to peeling off or cracking of the coating, which oftenhappens in the case of lacquer-coated cans produced according toconventional techniques. n

. Also important is the advantage of the instant process in causing muchless pollution of the environment, because the excess resin powder iseasily collected in the instant process, whereas the lacquer lost inconventional processes often ultimately pollutes the surrounding air orwater.

In a preferred aspect of this invention, the inner and outer surfaces ofthe articleto be coated, e.g., a can, are coated simultaneously. Thisaspect of the instant process can be implementedby providing a set ofspray nozzles which are synchronously inserted in the can, in rhythmwith the assembly line forwarding of the cans, which spray nozzles applyresin powder to the inner surfaces, while simultaneously a second set ofspray nozzles are provided for applying resin powder to the outsidesurface(s) of the article. By combining these two working steps, thearticles to be coated need be ex posed to a coating zone only once andalso require only a single heat treatment step. It is of course possibleto provide in the coating zone for multiple coats optionally withaltered resin particle grain size and/or constituency of the resinpowder used as the coating materials. The container to be coated may besuspended, in the coating operation, by way of a suction cup having lipsmade of heat-resistant Teflon or Viton of suitable shore hardness,fastened pneumatically to the outside bottom of the container, andwherein the container so held is grounded through an iron core containedin the suction cups. However, in a preferred aspect of the. apparatus ofthe invention, utilizable for carrying out the a. by way of steel prongsor needles which hold the container and simultaneously act to ground thecontainer, or, most preferably,

b. electromagnetically;

i. in the case of non-ferrous containers, e.g., aluminum cans, this isachieved by impressing into the outer bottom of the aluminum can an ironweb of, e.g., 1.5 'cm which serves to ground the container and permitselectromagnetic suspension described more fully hereafter (theimpressing of the iron web into the bottom of the aluminum can can beconveniently effected while the aluminum container itself is beingpressed or formed);

ii. in the case of cans or other hollow bodies made of ferrous metals,i.e., iron or steel (e.g., tin cans), by providing electromagneticsuspension to the ferrous article;

electromagnetic suspension is particularly preferred because itadvantageously provides both physical suspension or mounting of thearticle to be coated and simultaneous grounding of the'article. Further,

by regulating the magnetic field of theelectromagnetic suspension means(magnet) the suspension and release of the article to be coated can beclosely controlled and automatically regulated.

When making and processing aluminum cans in accordance with theinvention, the iron web adhered to, or wholly or partly imbedded in, thebottom of the aluminum cup is conveniently so adhered or imbedded duringcup formation, i.e., during the conventional impact extrusion processfor shaping aluminum cups from aluminum sheet. While the precise mode ofsupplying the iron webbing to the aluminum cup on formation thereof isnot narrowly critical, it is advantageous to dispose a roll of ironwebbing above the impact extrusion press and run the iron webbingthrough the extrusion assembly so that iron webbing is disposed underthe aluminum slug prior to punching of the cup therefrom; in thismanner, the iron webbing is adhered to the bottom of the cup on thepunch stroke of the extrusion press, i.e., simultaneously with cupformation, thus not requiring a separate step for adhering the webbing.It is, alternatively, of course possible, however, to adhere the ironwebbing subsequent to cup formation by any suitable mechanical means. Toperformapplication of the webbing during cup formation is howeverpreferred, and particularly also because the webbing so applied fallsoff during the conventional post-coating steps of strengthening (doming)the bottom of the coated can, during which steps the bottom of the canis rendered concave. Thus, the webbing applied to the flat cup bottom isso deformed on doming of the can bottom that it necessarily detachesitself from the can and can be discarded. It will be appreciated that asquare mesh is deformed during the doming operation to result inrhomboids being formed from the mesh squares, thus providing amechanical detaching action of the web from the cup bottom. The impactextrusion presses used in the "druwn-and-ironcd" process for making(two-piece) metal cans, in which iron webbing of the invention can beapplied, are conventional; see, e.g., Aluminum, published by theAmerican Society for Metals (1967 MetalsPark, Ohio; Vol. 2,'Chapter 7,and Volume 3, Chapter 4.

The'nortferrous, e.g., aluminum, can so provided with iron webbing, orthe steel can conventionally produced from tinplate, is then subjectedto the conventional steps of drawing, ironing, base forming andtrimming. Thereupon, the can is subjected to a washing or defattingoperation, which is substantially simpler in the process of thisinvention wherein resin powder is used to subsequently coat the can thanin processes wherein a liquid lacquer coating is applied, as explainedhereinabove.

After washing or defatting, the can is further processed in accordancewith the invention by suspending same, preferably electromagnetically,with equipment known in the art. Suitable apparatus is, e.g., thatavailable commercially from Fleetwood Systems, Inc, of Countryside,Illinois, which equipment is generally designated as magnetic elevators.In this manner, the cans are forwarded upside down suspended by only theoutside bottom portion and exposing all other outside surfaces and allinside surfaces of the can.

The so-suspended can is then forwarded to a coating zone in accordancewith the invention. In this coating zone, the suspended can is-subjectedto anelectrostat icdischarge field wherein the can, through the magneticsuspension thereof, acts as the ground and, while so subjected to theelectrostatic field, sprayed with finely divided resinous coating powdersupplied by spray nozzles preferably disposed both on the inside and onthe outside of the can. An advantageous embodiment of the coating zoneassembly and operation is illustrate in the drawing in which:

FIG. 1 is a schematic representation of the electro- FIG. 2 is aschematic flow-sheet of' (A) a prior art I process and (B) the inventiveprocess, for coating and printing cans.

With particular reference to FIG. 1, the cans are suspendedupside downfrom electromagnets which, in turn, are suspended from a conveyor beltor the like, shown in cross-section as member 17'; spray nozzles 13 aredisposed within each can and spray nozzles 15 are disposed outside thecan, to deliver resin powder to all can surfaces (except the outsidebottom which remains uncoated). The entire assembly is surrounded byantistatic container means 19 fitted with trough 21 through which looseexcess powder can be removed (and optionally recycled to the nozzles);the entire assembly is conveniently supported at some vertical elevationby frame 23.

The spray nozzles as well as the electrostatic field means within whichthey operate are conventional items of apparatus and are availablecommercially from, e.g., the Ransburg Electro-Coating Corporation ofIndianapolis, Indiana, and a typical overall installation is designatedas A 4.00 R-E-P Automatic by that corporation. As will be evident to theskilled artisan, the spray nozzles, which are an integral part of theelectrostatic discharge system, can be stationary or can be rotated forenhanced uniformity of powder distribution on the can.

The particular resin types which are employuble, in powder form, inaccordance with the invention are also, per se, known in the artalthough they have not been used to coat cans and'include, for instance,the

epoxy-esters, and epoxy-phenolics capable of being supplied in finepowder form by the spray nozzles used herein. It will be appreciatedthat the choice of a particular resin type will depend in part on theend use application of the can to be coated; e.g., if the can is anaerosol can for holding basic or acidic materials, the resin must beimpervious to attack therefrom.

It has been found that treated can surfaces in accordance with theinvention using an epoxy powder as the coating resin (applied at 180 to220C) demonstrated imperviousness to the following materials, whenexposed thereto, i.e., immersed therein, for twelve weeks at roomtemperature in a closed glass container.

Ammonia 10%) Sodium hydroxide (10%) Hydrochloric acid (10%) Acetic acid(10%) Citric acid (1%) Tartaric acid (1%) Oleic acid (conc.)

' Ethyl alcohol 50% Chlorothene NU Shaving foam (Gilette) Hair shampoocreme (Elidor and Schauma.) Hair coloring (Cone) Mustard Tomato pasteOlive oil Dist. water 7 Anti-freeze (Glysantin) Window wash liquid.

It can thus be seen that can surfaces treated by means of the inventiontolerate typical contents of, e.g., aerosol cans or food-containing cansas well as much more aggressive materials, to an outstanding extent.

After application of the resin powder on the can surfaces as describedabove, the cans are forwarded, by the electromagnetic conveyor to athrough-put oven wherein the cans aresubjected to a heat treatmentsufficient to melt the resin particles on the can surface, to form acontinuous film covering the entire inner and outer surface of the can.Advantageously, if the can is to be imprinted, as is usually the case,.the heat treatment is such that a continuous surface layer of a resinfilm is formed and partly dried, but is left sufficiently moist topermit imprinting in the wet-on-wet mode,

as discussed more fully hereinabove. Subsequent to imprinting, the canis desirably subjected to an additional heat treatment to firmly im bedthe print and to fully dry the resin coating to form a resilient andstrong protective sheath around the can. This can be advantageouslyachieved according to the invention by providing a single two-levelthrough-put oven wherein the cans are put through one level prior toimprinting and the second level after imprinting. The overallarrangement may be better understood byreference to P16. 2(B) whereinthere are shown, schematically but to approximate scale, two impactextruders (related equipment for drawing, ironing, base forming andtrimming, not shown) which deliver cans (e.g., aluminum cans having thebottom iron webbing adhered thereto) to washing equipment which isconnected through a short cooling zone to the coating zone wherein thecans are electrostatically sprayed with resin powder. The cans emergingfrom the coating zone are led through the lower level of a throughputoven which is of sufficient length to subject the can to a heattreatment (at, e.g., 180C) for about 5 minutes. The still moist coatedcan is then subjected to an offset printing operation whereafter theimprinted can is returned to the oven (now to the upper level) andsubjected to a finishing heat treatment firmly baking the print into thecoating and the coating onto the can. I

It can be appreciated, when comparing FIG. 2A (illustrating a typicalprior art process) with FIG. 2B herein that the space needs for theequipment required to carry out the instant process is less thanone-half of that demanded by prior art procesing, quite apart from thegreater adaptability of the instant process as regards processing ofaluminum cans (which of course are not processable with theelectromagnetic suspension techniques of the prior art). The use of thepowder resin, as opposed to liquid spray, for coating in accordance withthe invention permits a reduced washing operation and, in turn, ashorter cooling zone after washing and combination of inside and outsidecoating of the can in a single step (thereby eliminating an intermediatedrying step between the two coating steps of the prior art). The use ofpowder according to the invention is inherently more adapted to fast,continuousline processing than the use of liquid lacquer whichnecessitates waiting times, and elaborate drying steps after applicationof the liquid lacquer. Thus, the coating process of the instantinvention is capable of pro ducing a number of cans per unit time about50 percent higher than conventional processes while at the same time theinstant process requires less than half as much equipment space.Accordingly, one assembly according to the invention is capable ofhandling the output of, e.g., two impact extrusion presses where theprior art assemblies could process the can output of only one suchpress.

A particular advantage of the process of the invention resides in thepossibility that the electrostatic field can be reduced to such -a levelthat excess amounts of the coating powder are shaken off through avibrating means acting on the container, for instance as it is forwardedbetween the coating zone and the heat treatment zone. Because a certainpotential difference, i.e., voltage drop, is maintained between thecoating powder and the container surface, which, however, isre ducedwith increasing distance of the powder from the body surface, vibrationof the article causes principally those particles to fall off which arefarthest from the body surface, i.e., at the thickest places of thecoating. These shaken-off particles are led back, still within theelectrostatic field, to a collecting trough and then recycled to thespray nozzles. In this way, it is possible to fully recover all excesscoating powder, leading to substantial savings in raw material.

it has been found useful to also lead the coated article through meansfor tipping the container over, so that its open side points downwardlyat least part of the time (this is expediently done when the assemblychanges direction) to ensure that excess powder coating material withinthe container is shaken therefrom.

The following example is illustrative but not limitative of theinvention.

EXAMPLE Aluminum slugs weighing 18 grams (i.e.,corresponding in weightand diameter to the 45 mm diameter,

mm high cans to be formed) are delivered to a conventional' impactextrusion press. While each slug drops into the punch cavity of thepress, a piece of square iron webbing of 1.5 cm surface and a mesh sizeof l X 1 (being unwound from a roll disposed above the press) isplaced'behind the slug in such a manner that the punch stroke formingthe cup results in imbedding the piece of iron webbing into the outsidebottom of the aluminum cup being formed, as shown in FIG. 3 hereof whichis a schematic drawing of the web-inserting arrangement employed.

With specific reference to FIG. 3, the aluminum can, being extruded inthe impact extruder by extruder piston 33, is so disposed that onformation of the can its bottom end 31 receives web piece 41 which isembedded into the can bottom 31 by pressure of the extruder piston 33upon termination of the punch stroke of the piston. The web piece 41 isso embedded while in position (B). The iron webbing'is supplied fromroll 35 disposed above the impact extruder from which an iron webbingstrip 37 is intermittently unwound, and cut by web cutter 39 atappropriate length intervals. The resulting cut web piece 41 drops fromposition (A) to position (B) by gravity. The roll 35 and web cutter 39are geared into the overall assembly line to be synchronized with thestroke of the extruder piston in the impact extruder, so that onecut-piece of iron webbing 41 is supplied for each stroke, i.e., for eachcan formation. It is desirable to make the bottom end 31 of the cansomewhat thicker than the remainder of the can so that, upon doming thebottom of the can in subsequent processing, as shown in FIG. 3B, theresulting domed bottom will be of the same approximate thickness as theremainder of the can; thus, with reference to FIG. 3B,, the makingconcave of the can bottom 31 stretches the metal to thin it relative toits thickness during its former squared disposition. During the domingprocess, web piece 41 drops off and can be discarded (see FIG. 38 FIG.3A shown an advantageous embodiment of the iron webbing being unwoundfrom roll 35, i.e., containing small lateral guide pieces whichfacilitate alignment of the strip in the impact extruder to be properlyfitted to the middle of the can bottom. The width and length of the ironwebbing placed behind the slug is adapted to fit the bottom cupdiameter, and to be sufficiently large in relation to the weight of thecan to permit subsequent electromagnetic suspension of the can via theimbedded mesh. The mesh width and the strength (diameter) of theindividual filaments of the mesh are chosen to permit suspension butalso to not adversely affect or weaken the softer aluminum can bottom.For an aerosol can of cm bottom diameter, a piece of iron mesh of 1.5square cm surface was satisfactory.

The thus-formed can is washed with conventional media at 80C and dried(the drying step can be omitted if the washing is effected atsufficiently high temperatures, e.g., at over 100C) and then suspended(bottom mz. up) electromagnetically from a vertically adjustable(magnetic) conveyor belt fitted with' lateral guides to center cans ofvarying diameters on the belt. The so-suspended can, optionally stillwarm from washing, is introducedinto a coating zone comprisingconventional spray nozzles and an electrostatic field generator, thesuspended can forming the ground of that field. The can is sprayed withepoxy resin powder, ground to a very fineparticle size to achieve a thinfilm,

. 10 on both inside and outside surfaces simultaneously to a coatingthickness of 0.05 mm. 50 mz.

(In the electrostatic coating zone,- four cans can be simultaneouslycoated while suspended symmetrically in clover-leaf fashion with onenozzle disposed inside each of the cans and other nozzles symmetricallyarranged outside the cans). Still within the electrostatic field, thewheel mechanism of the magnetic conveyor belt is led over a cm. longtoothed rail, resulting in shaking of the suspended cans and shaking offof excess powder therefrom, thereby leaving on the can only thepredetermined amount of powder electrostatically adhering thereto. Theexcess powder is collected in a trough and recycled to the spraynozzles. The can is then immediately introduced into a drying oven (theprior art step of venting off vapors not being'required) as set outschematically in FIG. 2B. Shortly after being introduced into the oven,the cans are turned so that the open side now faces up by twisting ofthe conveyor belt. (This is done to avoid accumulation of resin materialat the lip of the cup, which would occur during the heat treatment ifthe can were still suspended upside down; such accumulation couldinterfere with subsequent placement of the can on a prong duringprinting). The residence time of the can in the lower pass of the ovenis 4 minutes at C and is such that the continuous film coating therebyformed on the cans is still moist enough to receive wet-on-wetimprinting. After leaving the lower pass of the oven, the can isreleased from its electromagnetic suspension by inter-' rupting theinduction current in the electromagnet means, and placed on theprecision prongs of a suitable printing press and imprintedconventionally. After imprinting, the cans are again electromagneticallysuspended and are led to the upper pass of the drying oven for 4 minutesat 180C whereby the printed matter is firmly imbedded into the filmcoating and the coating finally sintered. The can is now released fromthe electromagnetic suspension and finished, i.e., by necking-in andflanging. The bottom of the can is pressed inwardly (i.e. made concave)for better inside pressure strength of the can and, during this step,the iron mesh initially adhered to the can bottom falls off or is priedoff by the equipment used for doming the can bottom.

It will be understood that the specification and examples areillustrative but not limitative of the presentinvention and that otherembodiments within the spirit and scope of the invention will suggestthemselves to those skilled in the art.

We claim:

1. Process for producing imprinted aluminum cans from aluminum sheet,which process comprises:

a. forming a cup from the aluminum sheet while simultaneously adhering astrip of iron webbing to cover at least part of the bottom portion ofthe cup;

b. subjecting the thus formed cup, having said iron webbing adhered orpartly imbedded in its bottom portion, to a washing step to result insubstantial defatting of the cup;

' c. suspending the washed cup electromagnetically by means of the ironwebbing adhered thereto;

d. subjecting the thus-suspended cup to an electrostatic field whereinthe suspended can acts as the grounding element;

e. positioning nozzles capable of delivering particulated resinouscoating powder in spray form to the 2. Process as claimed in claim 1wherein two cupforming lines supply a single electrostatic coating zoneand there is at least one ofiset printing line to receive thefilm-covered can from said coating zone after said first heating step. I

3. Process as claimed in claim 1 wherein subsequent to said firstheating step (f) said can is mechanically deformed at the base thereofto form a concave bottom,

resulting in falling out of said iron webbing strip.

.F- Column 9, Line 59,

UNITED. STATES PATENT OFFICE CERTIFICATE OF CORRECTION June 11, 1974Patent No. 3 815 ,5 5 Dated lnventofls) Georg Becker and KarlKleindrettle It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

the wording o MP 1 should read "(bottom side up)-- Signed and sealedthis 17th day of September 1974.

(SEAL) Attest:

McCOY M. GIBSON JR. C. MARSHALL DANN Commissioner of Patents AttestingOfficer

1. Process for producing imprinted aluminum cans from aluminum sheet,which process comprises: a. forming a cup from the aluminum sheet whilesimultaneously adhering a strip of iron webbing to cover at least partof the bottom portion of the cup; b. subjecting the thus formed cup,having said iron webbing adhered or partly imbedded in its bottomportion, to a washing step to result in substantial defatting of thecup; c. suspending the washed cup electromagnetically by means of theiron webbing adhered thereto; d. subjecting the thus-suspended cup to anelectrostatic field wherein the suspended can acts as the groundingelement; e. positioning nozzles capable of delivering particulatedresinous coating powder in spray form to the outside and inside surfacesof the suspended cup in said electrostatic field; f. subjectiNg thesprayed cup having resin coating particles adhering thereto to a firstheat treatment to form a continuous film of molten resin particlescovering all sprayed surfaces of the can g. while the thus formed filmis still incompletely dried, imprinting the film covered can; and h.returning said imprinted cup to a second heat treatment to firmly bakeon the imprinted matter and to permanently dry the said film coating. 2.Process as claimed in claim 1 wherein two cup-forming lines supply asingle electrostatic coating zone and there is at least one offsetprinting line to receive the film-covered can from said coating zoneafter said first heating step.
 3. Process as claimed in claim 1 whereinsubsequent to said first heating step (f) said can is mechanicallydeformed at the base thereof to form a concave bottom, resulting infalling out of said iron webbing strip.